1. Field of the Invention
Embodiments disclosed herein relate generally to composite cutting structures.
More particularly, embodiments disclosed herein relate to polycrystalline diamond cutting elements formed to mitigate the residual stresses contained therein.
2. Background Art
Polycrystalline diamond compact (“PDC”) cutters have been used in industrial applications including rock drilling and metal machining for many years. In a typical application, a compact of polycrystalline diamond (“PCD”) (or other superhard material, such as polycrystalline cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide to form a cutting structure. PCD comprises a polycrystalline mass of diamond grains or crystals that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
PCD may be formed by subjecting a volume of diamond grains to certain high-pressure/high-temperature (“HPHT”) conditions in the presence of a sintering aid or binder. Conventionally, the sintering aid or binder is provided in the form of a solvent metal catalyst material, such as one or more elements from Group VIII of the Periodic table. The solvent metal catalyst may be added and mixed with the diamond grains prior to HPHT processing and/or may be provided during the HPHT process by infiltration from a substrate comprising the solvent metal catalyst as one of its constituent materials.
A conventional PDC cutter may be formed by placing a cemented carbide substrate into a HPHT container. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate in the container and the container is loaded into a HPHT device that is configured and operated to subject the container and its contents to a desired HPHT condition. In doing so, metal binder migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn bonded to the substrate. The substrate often comprises a metal-carbide composite material, such as tungsten carbide. The deposited diamond body is often referred to as a “diamond layer”, a “diamond table”, or an “abrasive layer.”
An example of a drag bit for earth formation drilling having PDC conventional cutters is shown in FIG. 1. In FIG. 1, a drill bit 10 has a bit body 12. The lower face of the bit body 12 is formed with a plurality of blades 14, which extend generally outwardly away from a central longitudinal axis of rotation 16 of the drill bit. A plurality of cutters 18 are disposed side by side along the length of each blade. The number of cutters 18 carried by each blade may vary. The cutters 18 are individually brazed to a stud-like carrier (or substrate), which may be formed from tungsten carbide, and are received and secured within sockets in the respective blade.
Conventional PCD includes 85-95% by volume diamond and a balance of the binder material, which is present in PCD within the interstices existing between the bonded diamond grains. Binder materials that are typically used in forming PCD include Group VIII elements, with cobalt (Co) being the most common binder material used.
Conventional PCD is stable at temperatures of up to 700-750° C., after which observed increases in temperature may result in permanent damage to and structural failure of PCD. In particular, heat caused by friction between the PCD and the work material causes thermal damage to the PCD in the form of cracks, which lead to spalling of the diamond layer and delamination between the diamond layer and substrate. This deterioration in PCD is due to the significant difference in the coefficient of thermal expansion of the binder material, which is typically cobalt, as compared to diamond. Upon heating of PCD, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the PCD. High operating temperatures may also lead to back conversion of the diamond to graphite causing loss of microstructural integrity, strength loss, and rapid abrasive wear.
In order to overcome this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure (either a thin volume or the entire body) to at least reduce the damage experienced from different expansion rates within a diamond-cobalt composite during heating and cooling. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a strong acid, typically nitric acid or combinations of several strong acids (such as nitric and hydrofluoric acid) may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC composite. By leaching out the cobalt, thermally stable polycrystalline (“TSP”) diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability with less effect on impact resistance. As used herein, the term thermally stable polycrystalline (TSP) includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by reinfiltrating the volume with a secondary material. An example of reinfiltration can be found in U.S. Pat. No. 5,127,923.
However, some of the problems described above that plague PCD cutting elements, i.e., chipping, spalling, partial fracturing, cracking or exfoliation of the cutting table, are also often encountered in TSP cutters or other types of cutters having an ultra hard diamond-like cutting table such as polycrystalline cubic boron nitride (PCBN) bonded on a cemented carbide substrate. In particular, it has been observed that TSP cutters are slightly more prone to spalling and delamination under severe loads. These problems result in the early failure of the cutting table and thus, in a shorter operating life for the cutter.
These problems, i.e., chipping, spalling, partial fracturing, cracking, and exfoliation of the PDC diamond layer may be caused in part by the difference in the coefficient of thermal expansion between the diamond and the substrate. Specifically, as shown in FIG. 5A, a cemented carbide substrate 53 has a higher coefficient of thermal expansion than a diamond layer 58. Thus, during sintering, for example, both the cemented carbide body 53 and diamond layer 58 are heated to elevated temperatures forming a bond between the diamond layer 58 and the cemented carbide substrate 53. As the diamond layer 58 and substrate 53 cool down, the substrate 53 shrinks more than the diamond 58 because of the carbide's higher coefficient of thermal expansion. Consequently, stresses referred to as thermally induced stresses, or residual stresses, are formed at the interface between the diamond and the substrate. Further, different contractions between the diamond layer and carbide substrate generate stresses in both bodies.
Moreover, as shown in FIG. 5B, residual stresses are formed on the diamond layer 58 from a mismatch in the bulk modulus between the diamond layer 58 and substrate 53. Specifically, the high pressure applied during the sintering process causes the carbide 53 to compress more than the diamond layer 58 due to the carbide's lower bulk modulus. After the diamond 58 is sintered onto the carbide 53 and the pressure is removed, the carbide 53 tries to expand more than the diamond 58 imposing a tensile residual stress on the diamond layer 58. These stresses may induce larger stresses, which may ultimately lead to material failure, because diamond and substrate materials typically have a high modulus (i.e., stiffness).
The cooling down effect shown in FIG. 5A (caused by different coefficients of thermal expansion) and the pressure release effect shown in FIG. 5B (caused by different bulk modulus) counteract with each other. As shown in FIG. 5C, the cooling down effect over powers the pressure release effect under commonly used sintering conditions, thereby leaving different net contractions in the diamond layer 58 and carbide substrate 53.
In an attempt to overcome these problems, many have turned to use of non-planar interfaces between the substrate and a PDC cutting layer. The belief being, that a non-planar interface allows for a more gradual shift in the coefficient of thermal expansion from the substrate to the diamond table, thus, reducing the magnitude of the residual stresses on the diamond. Similarly, it is believed that the non-planar interface allows for a more gradual shift in the compression from the diamond layer to the carbide substrate.
Additionally, the formation of a non-planar interface becomes more difficult to achieve when sintering a preformed diamond layer to a carbide substrate because any imprecision between mating non-planar surfaces of the diamond and substrate may cause cracking in the diamond layer.
Accordingly, there exists a continuing need for developments in cutting elements that possess reduced residual stresses therein.